The present invention relates to a surface inspection system for inspecting a very small foreign object or a very small flaw such as a crystal defect on a surface of a substrate such as a semiconductor wafer.
A surface inspection system is used for detection of a foreign object or a flaw on a surface of a substrate by projecting a laser beam on the surface of the substrate and by detecting scattered reflection light caused by the foreign object or the flaw. As a light emitting source for the surface inspection system, means such as a gas laser (He, Ar, etc.) has been used in the past. In recent years, a laser diode (LD) is used because of easiness to handle, safety, and longer service life, etc.
FIG. 27 represents a conventional type projecting optical system, using the laser diode as the light emitting source.
A laser beam 2 emitted from a light emitting source 1 is turned to a parallel beam by a collimator lens 3, and the beam is converged to a surface of a substrate 5 such as a wafer (a projecting point 18 at a converging position “f” by an image forming lens 4) by the image forming lens 4. The laser beam 2 is projected to the substrate 5 at an angle of θ. A scattered reflection light detector (not shown) detects a scattered reflection light from a position deviated from a reflection optical axis of the laser beam 2, e.g. from a direction approximately perpendicular to the paper surface. Distribution of a projected light intensity at the projecting point 18 is as shown in FIG. 28.
When the entire surface of the substrate 5 is to be inspected, the projecting point 18 is moved in a radial direction from the center to the outer edge of the substrate at a predetermined speed while the substrate 5 is being rotated. FIG. 28 shows distribution of the projected light intensity of the laser beam 2 at the projecting point 18. It represents a condition where the substrate 5 is rotated by one turn and the projecting point 18 moves from a scanning position “u” to a scanning position “u+1”. In this case, the speed of the projecting point 18 in the radial direction is the moving speed over a distance of “p” in the radial direction per one turn of the substrate 5.
Light amount of the scattered reflection light reflected by the foreign object or the flaw is influenced by the projected light intensity of the projected laser beam, and accuracy of detection of the foreign object or the flaw is also influenced by the projected light intensity of the laser beam. Therefore, in order to maintain a predetermined detection accuracy, it is necessary to have the projected light intensity higher than a predetermined light intensity I. The amount of movement “p” per one turn of the substrate as shown in FIG. 28 is determined so that the required projected light intensity I is maintained.
Wavelength of the laser beam 2 projected to the substrate surface is related to detection sensitivity and detection accuracy. By decreasing the wavelength or by increasing the projected light intensity, it is possible to improve the detection sensitivity. Therefore, by widening the range of projection while the projected light intensity is maintained at uniform level, it is possible to increase the detection accuracy while maintaining the detection sensitivity.
For quality control of a product, it is necessary to perform surface inspection of the substrate 5. For instance, in a process to manufacture a semiconductor product, a process of surface inspection for the substrate 5 is incorporated in the manufacturing process.
In recent years, density of semiconductor device is becoming higher, and there have been demands on the more improvement of detection sensitivity and detection accuracy of the surface inspection system. The surface inspection system is required to detect further very small foreign object or flaw on the surface of the wafer. Also, the inspection time required for surface inspection exerts influence on the throughput of the product, and it is desired to carry out the surface inspection quickly.
In the conventional cases as described above, when the projected light intensity of the laser beam is increased, peak value of the projected light intensity distribution is also increased. The detection sensitivity and the detection accuracy is improved by increasing the projected light intensity. In case the increase of the detection accuracy is not required, the amount of the movement for one turn of the substrate, i.e. scanning pitch “p”, is increased if the necessary projected light intensity is not changed. As a result, number of turns of the substrate necessary for the scanning of the entire surface of the substrate 5 is decreased, and this leads to the reduction of the time of surface inspection.
However, in case the laser diode is used as the light emitting source, despite of the fact that the laser diode has various advantages such as easiness to handle, safety and longer service life, there is such problem that it has lower light emission amount compared with the gas laser, etc., and there is a limitation to the increase of the projected light intensity. If the projected laser beam has shorter wavelength, the detection accuracy is improved. In this respect, the use of a laser diode emitting a blue laser beam with shorter wavelength is desirable. However, compared with a red laser diode, etc., the blue laser diode has further lower light emission amount, and it has such problem that enough light amount necessary for the surface inspection system can not be provided. To reduce the time for inspection, it is desirable to have wider projection range on the substrate surface. However, if the projection range is widened, the intensity of the projected light beam is decreased, and there has been such problem that both of detection sensitivity and detection accuracy become lower.
As described above, in the surface inspection system, the detection of the foreign object or the flaw is performed based on the detection of scattered reflection light. The scattered reflection light delicately changes according to the property of the substrate surface, i.e. type of film and film thickness. For instance, in case of a silicon oxide film (SiO2) which is formed on the surface of a silicon wafer, it is known that reflectivity changes according to film thickness. Also, it is known that the change of the reflectivity varies periodically according to film thickness, and the changes of the reflectivity is different also according to wavelength of the light.
FIG. 29 shows a reflectivity variation curve corresponding to film thickness for laser beams with 3 different wavelengths (488 nm, 680 nm, and 780 nm) when a silicon oxide film (SiO2) is formed on the surface of the substrate.
The sensitivity to detect the foreign object or the flaw is approximately correlated with reflectivity on the substrate surface. When the reflectivity becomes lower and light amount of the scattered reflection light is decreased, detection accuracy becomes lower. Therefore, in order to maintain a predetermined detection accuracy at a stable level, it is necessary to select the wavelength of the projected laser beam to match the type of film and film thickness.
In the conventional type surface inspection system as described above, it is necessary to set the wavelength of the laser beam to match the type of film and film thickness, and this adversely affects working efficiency. Further, the film thickness is not completely uniform for the entire surface of the substrate, and there is possibility that the reflectivity may vary according to each point of the substrate surface. When the reflectivity varies, there is possibility that the detection accuracy may vary.